You get loads of them, at least on Radio 4 comedies.
"We were promised Alana Titchmarsh as the star guest on my last cruise. We got someone from Torchwood instead." Ed Reardon's Week, tonight.
Fags, Mags and Bags: The local vicar is a Doctor Who fan. So is the local mullah. When they're both going to a convention, it's war: "I'm going as a Cyberman... if he hasn't bought all the tinfoil in Glasgow!" "Bit predicatable, Father? He's going as Mestor the Gastropod." "That SLUG!!!".
Down the Line: Spoof Radio 4 phone-in - the host has slowly been outed as a fan, and one episode last run ended with a five minute discussion about whether Logopolis was a sufficiently heroic end for the Tom Doctor...
And much more.